High voltage current limiting fuses are provided in high voltage distribution circuits to interrupt or isolate the circuit under fault current conditions. In order to provide positive interruption under these conditions, conductive end caps are mounted on each end of a fuse tube made of a very high grade of dielectric material so that it does not flash over when a high voltage is present across the end caps.
In this regard the two major problems confronting the dielectric material when used outdoors are ultraviolet rays from the sun and contaminants present in the atmosphere. It is generally recognized that ultraviolet rays can decompose organic dielectric materials which over time may decompose sufficiently to allow tracking and/or degradation of the mechanical properties between the end caps of the fuse. In the event of this type of failure flash over can occur between the end caps causing a disruption of the distribution circuit. Secondly, air contaminants over time can collect on the surface of the dielectric material resulting in a build up of materials which can be conductive when exposed to moisture. This can also lead to a flash over between the end caps also causing disruption of the distribution circuit.
These conditions have generally been resolved by increasing the length of the dielectric tube between the end caps of the fuse making flash over more difficult. However, in areas where severe contamination is present failure will still occur due to tracking. Efforts to overcome these problems have included the incorporation of mechanical devices which physically separate the terminals on disruption, thereby preventing any voltage stress from occurring across the dielectric material. Generally these devices allow the disconnected terminal to swing freely beneath the fuse which can be dangerous if it is at a high potential and comes close to a grounded object. These devices are also difficult to service in the field due to the difficulty in reconnecting the replacement element into the circuit.